Downside: It is "Windows only." That said, the editor runs in Parallels and Darwine just fine.

The runtime engine is another thing. There is a beta version of the Mac runtime that will run some games (like the Tierra games) just fine. But it won't run games made with the latest editor... yet. From what I've read, porting isn't much of a problem since it uses a cross platform library. But its not open source, so they're reliant on a single porter.

If you want to write SCUMM style games, I strongly suggest you look into it.